Circuits Assembly - February 2008 - (Page 22)

Packaging Trends Copper as a Viable Solution for IC Packaging By Sheila Rima C. Magno, Jean Ramos, Eduardo Pecolera and Chris Stai Not only more conductive and considerably cheaper than gold, copper uses the same ball bonding process. Ed.: For the complete article, visit circuitsassembly.com/cms/ content/view/6021. Table 1. 0.001” Copper and Gold Wire Properties3, 4 Cu Wire Electrical resistivity, µ -cm FAB hardness, Hv Ball bond hardness, Hv Tensile strength, gms Elongation, % 1.7 ~80 ~128 8-15 8-16 Au Wire 2.3 ~60 ~80 10-15 2-6 he most common IC conductor metals today are gold, aluminum, silver alloy and copper. Gold is the most widely used metal for IC wire bonding because of its resistance to surface corrosion and high productivity through the gold ball bonding process. However, the price of gold has risen more than 50% since June 2005, spurring interest in a replacement that could reduce cost without diminishing conductivity, chip functionality and reliability. Heavy copper wire, >0.002" diameter, has been used on power devices since 2001. 1 Aside from the lower cost, copper wire lowers electrical resistance, and increases the maximum allowable current and thermal stability at high temperatures. 2 For 0.001" copper wire development, the main driving factor is the replacement of 0.001" gold wire for cost-reduction purposes. Table 1 compares copper wire properties, setup requirements and reliability performance to those of gold wire. One of the most important properties is electrical conductivity. Based on the electri22 Circuits Assembly FEBRUARY 2008 T cal resistivity measured, 0.001" copper wire is ~26% more conductive than gold wire. Because of this, copper wire also offers the benefit of reducing bond pad size/die size by replacing current gold wire diameter with a smaller copper wire diameter at an equivalent performance. A well-known property of copper is its hardness. The copper free air ball (FAB) is already ~30% harder than a gold FAB. Finite element modeling studies have shown both copper and gold wire harden even more during ball bond deformation. 4 The change in hardness, ~20Hv, of gold during ball bonding is not significant enough to induce bond pad damage. However, the change in hardness, ~48Hv, of copper is enough to induce bond pad metal lift, or worse, silicon damage. Recent developments in copper bonding, like the availability of soft copper wires and the understanding of the effects of FAB parameters on ball hardness, have made it possible to overcome this. 5 ■ Ed.: This article was originally published at SMTA International in October 2007 and is reprinted here with permission. circuitsassembly.com http://circuitsassembly.com/cms/content/view/6021 http://circuitsassembly.com/cms/content/view/6021 http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - February 2008

Circuits Assembly - February 2008 Contents Caveat Lector Letters Industry News Market Watch Global Sourcing Better Manufacturing Maximizing Lean Copper As a Viable Solution for IC Packaging Embedded Active Components for High-Rel Products Cover Story: XRF Equipment As a RoHS Screening Tool Tech Tips Selective Soldering Test and Inspection Process Doctor Pb-Free Lessons Learned Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - February 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.